VGLUT3-p.A211V variant fuses stereocilia bundles and elongates synaptic ribbons

Abstract

DFNA25 is an autosomal‐dominant and progressive form of human deafness caused by mutations in the SLC17A8 gene, which encodes the vesicular glutamate transporter type 3 (VGLUT3). To resolve the mechanisms underlying DFNA25, we studied phenotypes of mice harbouring the p.A221V mutation in humans (corresponding to p.A224V in mice). Using auditory brainstem response and distortion product otoacoustic emissions, we showed progressive hearing loss with intact cochlear amplification in the VGLUT3^A224V/A224V mouse. The summating potential was reduced, indicating the alteration of inner hair cell (IHC) receptor potential. Scanning electron microscopy examinations demonstrated the collapse of stereocilia bundles in IHCs, leaving those from outer hair cells unaffected. In addition, IHC ribbon synapses underwent structural and functional modifications at later stages. Using super‐resolution microscopy, we observed oversized synaptic ribbons and patch‐clamp membrane capacitance measurements showed an increase in the rate of the sustained releasable pool exocytosis. These results suggest that DFNA25 stems from a failure in the mechano‐transduction followed by a change in synaptic transfer. The VGLUT3^A224V/A224V mouse model opens the way to a deeper understanding and to a potential treatment for DFNA25.

Key points

The vesicular glutamate transporter type 3 (VGLUT3) loads glutamate into the synaptic vesicles of auditory sensory cells, the inner hair cells (IHCs).

The VGLUT3‐p.A211V variant is associated with human deafness DFNA25.

Mutant mice carrying the VGLUT3‐p.A211V variant show progressive hearing loss.

IHCs from mutant mice harbour distorted stereocilary bundles, which detect incoming sound stimulation, followed by oversized synaptic ribbons, which release glutamate onto the afferent nerve fibres.

These results suggest that DFNA25 stems from the failure of auditory sensory cells to faithfully transduce acoustic cues into neural messages.

Spiking Pattern of the Mouse Developing Inner Hair Cells Is Mostly Invariant Along the Tonotopic Axis

During development, the sensory cells of the cochlea, the inner hair cells (IHCs), fire spontaneous calcium action potentials. This activity at the pre-hearing stage allows the IHCs to autonomously excite the auditory nerve fibers and hence, represents an efficient mechanism to shape the tonotopic organization along the ascending auditory pathway. Using calcium imaging, we show that the activity in the developing cochlea consists of calcium waves that propagate across the supporting and sensory cells. Both basal and apical IHCs were characterized by similar spontaneous calcium transients interspaced with silent periods, consistent with bursts of action potentials recorded in patch-clamp. In addition, adjacent auditory hair cells tend to have a synchronized [Ca²⁺]_i activity, irrespective of their location along the base-to-apex gradient of the cochlea. Finally, we show that the mechanical ablation of the inner phalangeal cells (IPCs), a class of supporting cells, reduces the synchronized [Ca²⁺]_i activity between neighboring sensory cells. These findings support the hypothesis that the tonotopic map refinement in higher auditory centers would depend on the synchronization of a discrete number of auditory sensory cells.

Maturation of ribbon synapses in hair cells is driven by thyroid hormone

Ribbon synapses of inner hair cells (IHCs) undergo developmental maturation until after the onset of hearing. Here, we studied whether IHC synaptogenesis is regulated by thyroid hormone (TH). We performed perforated patch-clamp recordings of Ca2+ currents and exocytic membrane capacitance changes in IHCs of athyroid and TH-substituted Pax8-/- mice during postnatal development. Ca2+ currents remained elevated in athyroid IHCs at the end of the second postnatal week, when it had developmentally declined in wild-type and TH-rescued mutant IHCs. The efficiency of Ca2+ influx in triggering exocytosis of the readily releasable vesicle pool was reduced in athyroid IHCs. Ribbon synapses were formed despite the TH deficiency. However, different from wild type, in which synapse elimination takes place at approximately the onset of hearing, the number of ribbon synapses remained elevated in 2-week-old athyroid IHCs. Moreover, the ultrastructure of these synapses appeared immature. Using quantitative reverse transcription-PCR, we found a TH-dependent developmental upregulation of the mRNAs for the neuronal SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) proteins, SNAP25 (synaptosomal-associated protein of 25 kDa) and synaptobrevin 1, in the organ of Corti. These molecular changes probably contribute to the improvement of exocytosis efficiency in mature IHCs. IHCs of 2-week-old athyroid Pax8-/- mice maintained the normally temporary efferent innervation. Moreover, they lacked large-conductance Ca2+-activated K+ channels and KCNQ4 channels. This together with the persistently increased Ca2+ influx permitted continued action potential generation. We conclude that TH regulates IHC differentiation and is essential for morphological and functional maturation of their ribbon synapses. We suggest that presynaptic dysfunction of IHCs is a mechanism in congenital hypothyroid deafness.